1. Field of the Invention
The present invention relates to a self-emulsifiable isocyanate-terminated prepolymer. More particularly, the present invention relates to a product which is suitable for an aqueous coating composition, and is suitable for an additive to an aqueous adhesive composition based on a water-soluble polymer and/or an aqueous emulsion.
2. Related Background Art
An example of a known self-emulsifiable polyisocyanate composition is a polyisocyanate composition which comprises (a) at least one aliphatic polyisocyanate and (b) a quantity of emulsifier which is sufficient to render component (a) dispersible in water or which is a reaction product of an aliphatic polyisocyanate with a nonionic polyalkylene ether alcohol as disclosed in U.S. Pat. No. 4,663,377. It is well-known that such self-emulsifiable polyisocyanate compositions are added to acrylic resin solutions, synthetic rubber solutions, polyurethane solutions, etc. for the purpose of improving water-resistance, heat-resistance, and adhesiveness.
Known self-emulsifiable polyisocyanate compositions, however, are disadvantageous in that the stability of the aqueous dispersion thereof and the chemical stability of the isocyanate group in the dispersion are not simultaneously achieved. In other words, when the polyisocyanate compound is modified to have higher affinity to water for the purpose of improving the dispersibilty in water, the isocyanate groups become more reactive to water. Thus, the isocyanate-group content in aqueous dispersion decreases with lapse of time. The decrease of the isocyanate-group content changes the properties of the composition employing the polyisocyanate compounds. On the contrary, when the polyisocyanate compound is modified to have longer pot life, the dispersibility of the compound in water becomes poor and the composition is not capable of improving sufficiently the properties of the composition employing the compound. Therefore, a self-emulsifiable polyisocyanate compound is strongly demanded which is sufficiently stable and usable for a long time in water dispersion.
After comprehensive study, it was found that both of the water dispersion stability of a polyisocyanate compound and the chemical stability of the isocyanate group in water are simultaneously achieved by introducing to the polyisocyanate compound a hydrophilic chain like a polyalkylene ether alcohol as conventionally conducted and additionally introducing thereto a lipophilic chain of an appropriate chain length in a well-balanced ratio to protect the isocyanate group surface-chemically. The present invention has been completed based on the above findings.